We don't have a tiny test or staging server we can slap this on to discover if it has the issues we experiences last time, do we?
By the way, I don't know if the "retroactive threadbanning" is avoidable, due to the simple logic of it all - If you are not allowed to see a user's content... then you can't see that content. If that content is the start of a thread... then you're not going to be able to see the thread. The fact that you were able to see it, and respond to it, previously doesn't really enter into it.
In some cases that may be annoying, or undesired, but by and large it is probably feature, rather than bug. Maybe, if you don't want to get locked out of a thread, you'd do well to pay attention how much you are cheesing people off...